Commodore 1084s-D1 High-pitch whine FIXED!

[Overkill]

Looking good!

I've found Q301 is short between C & E and D224 is reading as a 30 Ohm resistor.

Time to order some more components.

[Overkill]

I just found the attached, better late than never

[F0LLETT]

Thats better.

Seeing pic of board helps. Nice you found short.
Would be worth once you get parts, check for dry joints. I can see D224 has a nasty dry joint on one side.

Miss the days of faults, being just dry joints, . Judging by pic there are a few.

[Overkill]

Hands up, I was silly! 

Being too impatient to wait for replacement components, I installed all the components back except the faulty transistor and diode and switched the board on, I heard the flyback start-up for a few seconds and then it went quiet.  I switched it off and then back on and heard the flyback again, but after a few seconds C239 exploded!  :'(

C239 is rated at 50v, but it sounded like more than a 50v explosion to me (I'm not an expert )

I suppose looking at the diagram, that Q301 and D224 would take some of the load off C239.


 

[Overkill]

I've ordered 4.7uF 100v electrolytic non-polarised.

[F0LLETT]

I've ordered 4.7uF 100v electrolytic non-polarised.

Ouch, I would not have turned it on, until had replacement parts. You may have caused more damage.
Looking at small parts of the manual doesnt help. Where it 4.7uF located?

Edit: Nevermind I see it, around the Horizontal Drive circuit.

[Overkill]

Ouch, I would not have turned it on, until had replacement parts. You may have caused more damage.

Hopefully not, I will replace lots of caps and fit all the new parts before turning on again

[Overkill]

OK, I've replaced the bad components that I found and turned the monitor on, there was no BANG and no high-pitch whine

*** BUT *** (Information: I know very little about these monitors and not even used one for probably 30 years )

I believe I hear the high-voltage come on; it sounds a bit like a click, the LED comes on for a second and then goes off (Is that a bad sign?).

When I switch off the monitor, I hear the same click, which makes me think the high-voltage was still on.

I've seen nothing on the screen (I don't have anything connected).

If the LED stayed on, I'd assume I'd need to adjust the flyback to get a grey screen.

I'm going to wait for a little advice, as I'm not sure where to go from here.

[Overkill]

OK, the high voltage does start-up, I can feel the static on the screen, but then I think it goes off again.

[Overkill]

X-Ray protection maybe???

[Castellen]

the LED comes on for a second and then goes off (Is that a bad sign?).

The schematic shows that the LED is run from the regulated +12V supply, and that's derived from a 7812 linear regulator, and that's powered from a 15V unregulated supply that comes from the main power transformer.  So you've probably got some kind of a power supply issue.

Specifics around the power supply vary a lot between the many variants of the monitor, so I'd suggest narrowing down the correct schematic to begin with.  I've copied the other documentation on variants that you pointed out to my webserver so everything is in one place:
http://amiga.serveftp.net/schematics.html#1084

The obvious next step would be checking if the power transformer (not the line output transformer) primary is being switched or not, as if that's not happening then none of the various power supplies will be working.  Since it sounds like it starts working and then stops, the switching supply that drives the transformer primary might be going into some kind of shut-down protection state.  I'm guessing from here as the details of that circuit is completely different between model variants.

[Overkill]

@Castellen Thanks for the document link to your webserver, there's lots of interesting things on there

I think the schematics I've been using is pretty close to my monitor, it has a different H.O.T but maybe that could have been changed in the past.  The Flyback is labeled as T202 but mine is T201.
Also pin 4 of the flyback is not connected to earth like mine, but I think that must be an error in the diagram, as I think pin 4 would be part of the X-Ray protection system.

I was thinking along the same lines as you, that it could be a power supply issue, but the more I thought about it and the history of this repair:>
>The monitor was working well until it started the high-pitch-whine and smoking.
> Replaced H.O.T circuit components and now get high-voltage to the screen for a second.
It made sense that it could be the X-Ray protection, so I disconnected pin 8 of IC201 (the X-Ray pin) and tried the monitor on reduced voltage.

I reduced the voltage with a very crude home-made current protection device, made out of a light dimmer switch and a 60w light bulb

The monitor powered up and stayed on, the LED was flashing but I think that was just because it was running on low voltage through my very crude current restrictor.

When I cut the power, I got a very bright white spot on the screen for a second or two.

I've not tried it on full mains yet, because I don't have a lead X-Ray proof suit

So from that experiment it must be the X-Ray protection circuit that was cutting off the monitor.

One thing I never mentioned was that the original flyback was a DCF-1580, but I replaced it with what is supposed to be a compatible MSH1FCT31.

After some reading-up about this type of protection circuit on CRTs, it says it can be cause by:
> a capacitor in the HOT circuit that's changed in value (I will check or replace them).
> a faulty Flyback (maybe it got damaged because it was switched on for a second with those faulty components)
> wrong B+supply voltage (Wait! thinking-outloud I think I tested this and it was 109v, I was thinking it should be 110v so thought it was ok, but now I see it should be 105v??)

I did have more to say here, but now I think I better check that B+supply again, that is the voltage that goes to the HOT circuit winding of the flyback? pin 7 on my diagram, right???  And can be adjusted somewhere?

Thanks for sticking with this, maybe I might need to get help from a TV CRT repair group, as messing around with this monitor with protection disabled is a bit scary lol

[Overkill]

I see someone had the same problem with a MSH1FCT31
https://forum.amiga.org/index.php?topic=72857.msg831657#msg831657

[Overkill]

Confusion  the circuit diagram I'm using has a DCF-1580 flyback which is what I've removed from my monitor, BUT the diagram says 105v to pin 7, BUT on the underside of my monitors board it says 110v to pin 7?  That would explain the X-Ray protection, also I've seen some smoke which I think was coming out the flyback replacement

[Castellen]

I wouldn't get too carried away with the 105V vs 109V vs 110V, it's hardly going to be exact.

If you did see smoke from the transformer, it could well have been damaged due to the other issue you've since found and fixed.  When you've got a lot of DC current going through a transformer like that, it usually doesn't end well for for the transformer.

I'd suggest doing a bit more research to be absolutely sure that the replacement line output transformer was compatible.  I've previously seen some helpful discussions on various forums around model numbers and compatibility.  Then try again with another transformer.

[Overkill]

I shouldn't have listened to Google, everywhere it said "high-pitch whine = bad flyback", but the original flyback was fine

So, it turned out to just be that transistor in the EW circuit and the diode in the HOT circuit

It sounds like I could have used the new flyback, but I would have had to change the value of the tuning capacitor in the HOT circuit.

Thanks for all the help, it's much appreciated